R.I. Zakharchenya

Radiative transitions and spectral-hole burning in MgO: Cr3+ nanocrystals

Luminescence spectra of Cr3+ ions in nanocrystalline MgO obtained by sol-gel technology were studied. The radiative lifetime of excited 2E states of cubic and tetragonal Cr3+ centers was found to be considerably longer than that of bulk single crystals. This effect is caused by a modified effective refractive index of the inhomogeneous medium. Burning of long-lived spectral holes in the 4A2-2E transition profile of cubic centers was observed, which sets nanocrystalline samples strongly apart from bulk crystals, where no hole burning was found.

Selective laser spectroscopy of RE3+ and Mn4+ in sol-gel technique produced Al2O3

Sol-gel produced nanocrystalline highly porous transparent y-Al 2 O 3 doped with RE and TM ions and α-Al 2 O 3 ceramics, produced by its annealing, were studied with selective laser spectroscopy techniques. The site-selective spectroscopy of RE 3+ ions in α-Al 2 O 3 reveals that these ions form centers in corundum with one dominating type. The FLN studies of homogeneous line widths of 5 D 0 - 7 F 0.1 transition in γ- and α-Al 2 O 3 extend the low-temperature hole-burning data (Feofilov et al., Phys. Rev. B 54 (1996) 3690). It is shown that Mn 4+ -doped Al 2 O 3 is potentially effective photochemical hole-burning material and self-gated persistent hole-burning in α-Al 2 O 3 : Mn was demonstrated.

Luminescence and laser spectroscopy of highly porous γ-Al2O3 doped with 3d- and 4f-ions: Effects of spatial phonon confinement

Abstract Inhomogeneously broadened luminescence spectra of Cr3+, Eu3+ and Pr3+ ions in sol-gel-grown monolithic highly porous γ-Al2O3 were observed. In FLN and HB experiments, the interaction of ions with the size-resonant terahertz acoustic vibrations of crystalline nanoparticles which form the porous network was studied. The incorporation of Eu3+ and Pr3+ into α-Al2O3 (corundum) lattice is demonstrated in annealed (T > 1200 °C) sol-gel-prepared samples.

Optical generation of nonequilibrium terahertz resonant vibrational excitations in highly porous aluminium oxide

Abstract The dynamics of terahertz phonons generated at liquid helium temperature by laser excitation in sol-gel technology grown transparent highly porous crystalline γ-Al2O3 samples, was studied through the phonon-induced anti-Stokes FLN spectra of Cr3+ impurity ions (2E-4A2 lines). The generated phonon spectrum contains essentially a nonequilibrium narrow peak around ω ∼ 0.6 THz (20 cm−1) with high occupation numbers n ω and decay time ~ 1 ms besides and a slowly cooling (∼tens of ms) thermalized Planckian distribution. The frequency (20 cm−1) of these long-living nonthermalized vibrations is consistent with the lowest acoustic, size-resonant surface mode of γ-Al2O3 spherical nanoparticles with 80 A diameter, which form the network of the sample. Discussed are the nature of the long lifetime and the possible role of these surface modes in the spectral dynamics of optically generated phonons.

Transient hole-burning and homogeneous linewidth of Eu3+ in sol-gel-produced corundum

In α-Al 2 O 3 , small-grain ceramics prepared by annealing of sol-gel-produced γ-Al 2 O 3 , it is possible to obtain trivalent rare earth ions at a single regular site despite the large difference in RE 3 and Al 3 + ionic radii. Transient spectral holes, generated by a redistribution of the population among the hyperfine levels of the two isotopes of Eu 3+ , were burned in the inhomogeneously broadened lineshape of the 7 F 0 - 5 D 0 transition of Eu 3+ . This observation gives additional evidence for the regular structure of Eu 3+ centers in small-grain corundum ceramics. The temperature-dependent hole broadening probably results from dephasing of electronic states by two-phonon Raman processes. The results are discussed together with earlier non-resonant fluorescence line narrowing results on the 5 D 0 - 7 F 1 transition, obtained by single frequency excitation of the 7 F 0 - 5 D 0 transition.

Electron paramagnetic resonance of Er3+ ions in a polycrystalline α-Al2O3

The EPR spectra of rare-earth Er3+ ions in a polycrystalline corundum α-Al2O3 synthesized by the sol-gel technology were revealed. It is shown that the EPR spectra belong to the Er3+ ions in the ground state corresponding to the lower Stark sublevel of the 4I15/2 term and can be described by the spin Hamiltonian of axial symmetry with an effective spin S = 1/2 and the g tensor with components g‖ = 12.176 and g⊥ = 4.14. The average value of the g tensor (〈g〉 = 6.82) corresponds to the Γ7 state in a cubic field. Erbium is assumed to substitute for aluminum in the Al2O3 corundum crystal. The local symmetry C3 of the Al3+ ion remains despite the pronounced expansion of the lattice around the Er3+ ion.

Magnetic resonance in PbxNbyOz ceramics as a system containing chemical fluctuation regions

Simulation of the chemical fluctuation regions in PMN-like relaxors through growth of the PbxNbyOz eramics was performed. Different PbxNbyOz clusters (chemically and structurally) coexist in such ceramics. Hole polaron and bipolaron (Cr3+-two polaronic-hole) paramagnetic complexes were considered for explanation of the EPR spectra in PbxNbyOz ceramics. Dynamical averaging, light-induced effects, and significant effects of reduction treatment giving the coexistence of Nb5+ and Nb3+ ions, as well as of a strong internal magnetic field, were discovered in this ceramics. The latter could be related to antiferromagnetic phase realization in PbxNbyOz clusters containing a sufficiently high concentration of magnetic Nb3+ host lattice ions. Such a situation leads to antiferromagnetic resonance on Nb3+ ions, as well as to EPR of Cr3+-related paramagnetic complexes in a Nb3+-induced internal magnetic field. Charge transfer vibronic excitons (CTVE) in free and in CTVE phase states were detected in PbxNbyOz ceramics by photoluminescence studies.

Polarons and charge-transfer vibronic excitons in ferroelectric oxides: EPR and photoluminescence studies of Pb x Nb y O z ceramics

We performed a simulation of the chemical fluctuation regions in the lead-magnesium-niobate-like relaxors by growing PbxNbyOz ceramics. The coexistence of PbxNbyOz clusters with different chemical composition that is inclusive of Nb5+ as well as Nb3+ ions was assumed. The existence of mobile polaronic pseudo-Jahn-Teller holes on the oxygen ions in different PbxNbyOz clusters has been suggested by the electron paramagnetic resonance (EPR) experiment. Here light- induced EPR and EPR motional averaging effects related with such polaronic holes have been manifested. The appearance of new paramagnetic centers (S=5/2, 3/2, 1/2) related with Cr3+ impurities at the Nb5+ sites with trapped hole pairs (hole bipolarons), the charge compensation and the exchange effect have been proposed as explanation for the results of the EPR experiment. The charge-transfer vibronic excitons with O → Nb5+ charge transfer were directly detected by photoluminescence studies. The R-line in the Cr3+ impurity photoluminescence evidences the appearance of the Cr3+ ion in such matrices.